This invention relates to a tracking servo system for tracking the center of a track with a light spot, and more particularly to a composite tracking servo system in which a differential diffraction method using pregrooves or a so-called push-pull tracking method is combined with a wobbled tracking method using wobbled track marks disposed in a relation wobbled relative to the center of a track, and which is suitable for application to an optical code data memory such as a recordable/reproducible adding (write once) type optical disc system or an erasable type optical disc system.
In the push-pull tracking method, an optical disc having guide grooves or so-called pregrooves formed previously along the direction of rotation of the disc is irradiated with a light spot, and an unbalance of the distribution of the reflection of light diffracted from the pregrooves is based to detect a track error which is fed back to a servo system. This push-pull tracking method is disclosed in, for example, U.S. Pat. No. 4,363,116. Since this push-pull tracking method utilizes the distribution of the light diffracted from the pregrooves, an offset component attributable to an eccentricity or tilt of the disc tends to occur, and, because of such an offset component, the light spot cannot be accurately positioned on the center of the track. According to the researches made by the inventors, a tilt of 0.7.degree. or an eccentricity of 100 .mu.m, for example, results in an offset of about 0.1 .mu.m.
On the other hand, a tracking servo system of three spots type is widely employed in a playback-only system such as a CD (a compact disc). However, this tracking servo system is unfit for application to a combined recording/reproduction system. The tracking servo system of three spots type is disclosed in, for example, U.S. Pat. No. 3,876,842.